In conventional offshore pipelay operations, plugs are welded onto the pipeline end when the end must be laid on bottom. This can be done as planned or emergency measure, permitting the pipeline to remain dry and light for subsequent recovery of the pipeline end. The welded plug both seals the pipe end to keep out water and provides the means for transferring tension to the pipe during abandonment and recovery. After recovery the welded plug may simply be cut off.
For deepwater pipeline construction operations where subsea connections are required, there is a need for different kinds of plugs. Like welded plugs, such plugs must keep out water to lighten the pipe and ease alignment operations, but the plugs are not necessarily used for pulling, since there may be additional alignment structure at the pipe end which can serve as a pull point. The major different is that once alignment of pipe connectors is achieved, the plugs must be removed on bottom in conjunction with the connection operations rather than being brought to the surface for removal.
Somehow the hydrostatic pressure across the plugs must be equalized, and the plugs safely released just before or after the connectors are joined. It is preferable to perform the plug release and removal operation remotely, that is, without divers, since diving operations in deep water become prohibitive both technically and economically.
From the foregoing, it is clear that welded plugs would be difficult to use in deep subsea connection. Even if the welded plug could be removed on bottom (perhaps explosively), there is then a need in the art for a suitable pipe end configuration for subsequent connection operations.